Recent research in the area of purine nucleoside phosphorylase (PNP), methylthioadenosine phosphorylase (MTAP), 5′-methylthioadenosine nucleosidase (MTAN), and nucleoside hydrolase inhibitors has resulted in the design of a class of compounds known as the Immucillins, some of which are potent inhibitors of one or more of the above enzymes. Immucillins are nucleoside analogues where the “sugar” part of the molecule has been replaced with an “imino sugar” moiety.
PNP catalyses the phosphorolytic cleavage of the ribo- and deoxyribonucleosides of guanine and hypoxanthine to give the corresponding sugar-1-phosphate and guanine, hypoxanthine, or other purine bases.
Humans deficient in PNP suffer a specific T-cell immunodeficiency due to an accumulation of dGTP which prevents stimulation of T lymphocytes. Inhibitors of PNP are therefore immunosuppressive, and are active against T-cell malignancies and T-cell proliferative disorders.
U.S. Pat. No. 5,985,848, U.S. Pat. No. 6,066,722 and U.S. Pat. No. 6,228,741 describe compounds known as Immucillins that are inhibitors of PNP and purine phosphoribosyltransferases (PPRT). These Immucillins are useful for treating parasitic infections, T-cell malignancies, autoimmune diseases and inflammatory disorders. They are also useful for immunosuppression in organ transplantation.
U.S. Pat. No. 6,693,193 and U.S. Pat. No. 7,022,852 describe a process for preparing certain Immucillin compounds, providing another useful route to the synthesis of this class of compounds. U.S. Pat. No. 7,109,331 discloses further Immucillins that are inhibitors of PNP and PPRT.
The imino sugar part of an Immucillin molecule has the nitrogen atom located between C-1 and C-4 so as to form a 1,4-dideoxy-1,4-imino-D-ribitol compound. The location of the nitrogen atom in the ribitol ring may be important for binding to enzymes. In addition, the location of the link between the imino sugar moiety and the nucleoside base analogue may be critical for enzyme inhibitory activity. The compounds described above have that link at C-1 of the imino sugar ring.
Another related class of nucleoside phosphorylase and nucleosidase inhibitor compounds, known as DAD-Me-Immucillins, has been developed. The location of the nitrogen atom in the imino sugar ring of this class of compounds is varied and/or the imino sugar moiety is linked to the nucleoside base analogue via a methylene bridge. DAD-Me-Immucillins are described in U.S. Ser. No. 10/524,995.
Some of the Immucillins have also been identified as potent inhibitors of MTAP and MTAN. These are the subject of U.S. Ser. No. 10/395,636. MTAP and MTAN function in the polyamine biosynthesis pathway, in purine salvage in mammals, and in the quorum sensing pathways in bacteria. MTAP catalyses the reversible phosphorolysis of MTA to adenine and 5-methylthio-α-D-ribose-1-phosphate (MTR-1P). MTAN catalyses the reversible hydrolysis of MTA to adenine and 5-methylthio-α-D-ribose, and the reversible hydrolysis of S-adenosyl-L-homocysteine (SAH) to adenine and S-ribosyl-homocysteine (SRH). The adenine formed is subsequently recycled and converted into nucleotides. The only source of free adenine in the human cell is a result of the action of these enzymes. The MTR-1 P is subsequently converted into methionine by successive enzymatic actions.
MTA is a by-product of the reaction involving the transfer of an aminopropyl group from decarboxylated S-adenosylmethionine to putrescine during the formation of spermidine. The reaction is catalyzed by spermidine synthase. Likewise, spermine synthase catalyses the conversion of spermidine to spermine, with concomitant production of MTA as a by-product. Spermidine synthase is very sensitive to product inhibition by accumulation of MTA. Therefore, inhibition of MTAP or MTAN severely limits the polyamine biosynthesis and the salvage pathway for adenine in the cells.
MTA is also the by-product of the bacterial synthesis of acylated homoserine lactones from S-adenosylmethionine (SAM) and acyl-acyl carrier proteins in which the subsequent lactonization causes release of MTA and the acylated homoserine lactone. The acylated homoserine lactone is a bacterial quorum sensing molecule in bacteria that is involved in bacterial virulence against human tissues. The homoserine lactone pathway will suffer feedback inhibition by the accumulation of MTA.
MTAP deficiency due to a genetic deletion has been reported with many malignancies. The loss of MTAP enzyme function in these cells is known to be due to homozygous deletions on chromosome 9 of the closely linked MTAP and p16/MTS1 tumour suppressor gene. As the absence of p16/MTS1 is probably responsible for the tumour, the lack of MTAP activity is a consequence of the genetic deletion and is not causative for the cancer. However, the absence of MTAP alters the purine metabolism in these cells so that they are mainly dependent on the de novo pathway for their supply of purines.
MTAP inhibitors are also expected to be very effective against parasitic infections, such as malaria which infects red blood cells (RBCs), because such infections lack the de novo pathway for purine biosynthesis. Protozoan parasites depend entirely upon the purines produced by the salvage pathway for their growth and propagation. MTAP inhibitors will therefore kill these parasites without having any negative effect on the host RBCs, because RBCs are terminally differentiated cells and they do not synthesize purines, produce polyamines or multiply.
MTA has been shown to induce apoptosis in dividing cancer cells, but to have the opposite, anti-apoptotic effect on dividing normal cells such as hepatocytes (E. Ansorena et al., Hepatology, 2002, 35: 274-280). Administration of MTA in circumstances where its degradation by MTAP is inhibited by an MTAP inhibitor will lead to greater circulatory and tissue levels of MTA and consequently an enhanced effect in the treatment of cancer.
MTAP and MTAN inhibitors may therefore be used in the treatment of diseases such as cancer, bacterial infections or protozoal parasitic infections, where it is desirable to inhibit MTAP or MTAN. Such treatments are described in U.S. Ser. No. 10/395,636 and U.S. Ser. No. 10/524,995.
The Immucillins and DAD-Me-Immucillins are also useful as inhibitors of nucleoside hydrolases. These enzymes catalyse the hydrolysis of nucleosides. They are not found in mammals, but are required for nucleoside salvage in some protozoan parasites. Certain protozoan parasites use nucleoside phosphorylases instead of, or as well as, nucleoside hydrolases for this purpose. Inhibitors of nucleoside hydrolases and phosphorylases can be expected to interfere with the metabolism of the parasite and therefore be usefully employed against protozoan parasites.
The X-ray crystal structure of one of the inhibitor compounds (DAD-Me-Immucillin-H) bound to Mycobacterium tuberculosis PNP has been described (A. Lewandowicz, W. Shi, G. B. Evans, P. C. Tyler, R. H. Furneaux, L. A. Basso, D. S. Santos, S. C. Almo and V. L. Schramm, Biochemistry, 42 (2003) 6057-6066.). The complex of this inhibitor with PNP has favourable hydrogen bonds to almost every hydrogen bond donor-acceptor site in the complex. Even a slight structural change can disrupt this favourable hydrogen bonding pattern, as demonstrated by energetic mapping of transition state analogue interactions with human and Plasmodium falciparum PNPs (A. Lewandowicz, E. A. T. Ringia, L.-M. Ting, K. Kim, P. C. Tyler, G. B. Evans, O. V. Zubkova, S. Mee, G. F. Painter, D. H. Lenz, R. H. Furneaux and V. L. Schramm, J. Biol. Chem., 280 (2005) 30320-30328).
It was previously considered that, in view of the importance of hydrogen bonding and the location of chemical moieties in the donor-acceptor site, an inhibitor of these enzymes would likely require the imino sugar moiety to have a 5-membered ring and to have chirality at certain locations. However, in the ongoing search for new and improved nucleoside phosphorylase and nucleosidase inhibitors, the applicants found that azetidine analogues of Immucillins and DAD-Me-Immucillins, which have a 4-membered ring as the imino-sugar analogue, some of which are achiral, are surprisingly potent inhibitors of at least one of PNP, PPRT, MTAP and MTAN. The 4-membered ring of the azetidine would not have been expected to orient functional substituents such as hydroxyl-groups in orientations close enough to effectively participate in the hydrogen-bonding networks considered responsible for the potent inhibition observed for the Immucillins and DAD-Me-Immucillins.
It is therefore an object of the present invention to provide novel inhibitors of PNP, PPRT, MTAP, MTAN, and/or nucleoside hydrolases, or to at least provide a useful choice.